To the outside world it has become as known as Africa's First World War with its foreign armies and invasions, and ceaseless killing and dying that seems to achieve nothing. The battleground is the east of the Democratic Republic of Congo, where to some of those who have fought it is a matter of the very survival of nations, while to others it is the prospect of immense wealth that drives them. But what are the true roots of this conflict, and what keeps it alive?

Little more than a decade ago, the long-suffering citizens of the Democratic Republic of Congo could not have imagined their situation could get much worse.

Their brief flirtation with hope and rebirth after independence from Belgium in 1960 soon gave way to decades of decline under the derelict rule of Mobutu Sese Seko. He renamed the sprawling central African state Zaire, cloaked himself in leopard skins and African nationalism, and set about filling his bank accounts while most of those he ruled saw their country crumble around them.

Through the 1970s and 1980s, Zaire fractured into a series of city states increasingly distant from each other as roads were worn away by neglect, telephone systems collapsed and postal services ceased.

In major cities such as Kisangani, in the north, conventional administration largely fell away. The justice system disappeared and traditional courts re-emerged out of necessity. Border towns, such as Goma in the east and Lubumbashi in the south, effectively became economic appendages of neighbouring countries. Across Zaire, hospitals, lacking in medicines and trained staff, were places merely to go and die.

The elderly can describe a time when cars travelling the length of the continent, from South Africa or the Rhodesias to Cairo or Lagos, passed through Lubumbashi and Goma, their passengers stopping at the best hotels on Lake Kivu. A couple of decades later, it was impossible for any ordinary car to make it more than a short distance out of either city and the hotels had long since fallen apart, their swimming pools dry and rooms infested with cockroaches.

Others tell of the self-delusions of the early 1970s when Mobutu's rule really did seem to offer the promise of Zaire as the country of tomorrow. A new elite of educated professionals – economists, businessmen, even nuclear scientists – gave up good jobs in Europe and America to return home to build the dream, and came to regret it.

By the 1990s, and the last years of Mobutu's dictatorship, there were no such illusions. People said the sooner the old kleptocrat went the better because whatever followed could not be any worse.

But then came the Rwandans and a decade of war, mass death and, in large parts of Congo, the destruction of what remained of functioning government. This turmoil has changed the face of central Africa, shifting the balance of power to the tiny country of Rwanda, once scorned as backward and irrelevant but now feared and loathed by its larger Congolese neighbour – and sometimes compared with Israel for its actions and purpose as an entire region has been plunged in to conflict.

Exodus from Rwanda

The roots of Congo's grief and central Africa's upheaval lie in the assassination of Rwanda's president, Juvenal Habyarimana, in April 1994. With Habyarimana out of the way, a Hutu extremist regime seized power and set about murdering 800,000 Tutsis in the last genocide of the 20th century.But while the regime was good at killing unarmed civilians it proved less efficient against a Tutsi rebel group, the Rwandan Patriotic Front (RPF) under Paul Kagame, which drove the extremists from power and with it sent more than a million Hutu refugees fleeing into Zaire fearing revenge for the genocide.

There they met what some deemed to be a fitting punishment, dying in their tens of thousands of cholera on the desolate volcanic rock around Goma under a sky darkened by ash spewing from the rumbling volcano. Mass graves were filled with women, children and the elderly.

That might have been the end of a great tragedy but with the civilians pouring into Zaire had come their political leaders, defeated soldiers and Hutu militiamen – the interahamwe – who had led the genocide back in Rwanda. Their presence, and Mobutu's continuing support for his old Hutu allies, sowed the seeds of much of the upheaval that was to come and the Zairean leader's own downfall as the new Tutsi-led government in Rwanda vowed not to stand idly by while another genocide was prepared next door. The United Nations played an important part in this. The UN High Commissioner for Refugees (UNHCR) took the fateful decision to recognise the Hutu extremists as leaders of the refugee camps and gave them control of food distribution. That ensured that the military men remained well fed and fit, and gave them considerable control over the sprawling camps which were soon transformed into armed bases to continue the war against the new government in the Rwandan capital, Kigali.

The extremists infiltrated Rwanda, planting mines and massacring civilians. They hauled people off buses, separating the Tutsis and then shooting them, and forcibly recruited young Hutu men into their ranks.

Two years later, in 1996, Rwanda decided it could no longer tolerate the camps keeping the threat of genocide alive on its border and it invaded Zaire. The Tutsi-dominated army surrounded the refugees and drove hundreds of thousands of people back across the border.

Those who went home were largely unmolested but those remaining in Zaire, who included the former soldiers and militiamen and their wives and families, were remorselessly perused and slaughtered. The bodies of women and children were dumped in the mass graves left by the Rwandan army and its Congolese rebel front led by Laurent Kabila.

One of those who stayed in Zaire was Edmond Ngarambe, a lieutenant in the defeated Rwandan Hutu government's army. His journey over the following years was typical of the wandering, fighting existence of the Hutu exiles. He was in a refugee camp in Bukavu when the new Rwandan army – what had been the Tutsi rebel force that defeated his own – invaded.

Ngarambe fled east, deeper in to Zaire, with his wife and siblings. "I walked with my family to Tingi Tingi. I saw a lot of people die on the way. My brother was one of them. We started walking on 28 October 1996 and we arrived on the 25 December," he said.

But even Tingi Tingi, deep inside Zaire and hundreds of kilometres from the Rwandan border, proved no refuge. His enemies were still approaching. Mobutu organised an airlift of the defeated Hutu Rwandan army soldiers there to Kisangani on the banks of the Congo river.

"I was wounded in the war and tried to leave the army but my commanding officer wouldn't let me," said Ngarambe.

It wasn't long before Kisangani, too, fell to the Rwandan army and Kabila. Ngarambe fled just ahead of them, taking a boat down the river and then walking into the Central African Republic, where he was to stay for eight months.

"Our enemies were chasing us everywhere. I had to chose the path to resist because I didn't have anywhere to go. Congo was in the hands of Rwanda, it was at the mercy of Rwanda," he said. The Rwandan invaders reached their neighbour's capital city, Kinshasa in May 1997. Right to the end Mobutu could not fathom how it was that tiny Rwanda had toppled his once monolithic regime. He failed to understand the drive of the Tutsis in their determination to pursue the murderers of their people, and to protect the power they now wielded.

Mobutu also failed to grasp just how much the world had changed with the end of the Cold War, and how France and America would no longer protect his regime. He was forced to flee, and the Rwandans installed Laurent Kabila in his place in Kinshasa. Kabila promptly changed his country's name from Zaire back to the Democratic Republic of Congo.

Paul Kagame, then Rwanda's Tutsi vice-president but the real power in the land as the head of the army that had overthrown his country's Hutu regime, saw Kabila as his man in Kinshasa who would help crush the Hutu extremists and protect Rwanda's borders. But Kabila proved to be less of a puppet than the Rwandans hoped, and certainly less interested in dealing with the problem of the Hutu genocidaires than they expected. The two soon fell out. So Rwanda invaded again in 1998 intent on creating a buffer zone extending hundreds of miles from its border into Congo, and installing a client regime to administer it.

Ngarambe found himself inducted into the Congolese army, fighting for the man he had previously fought against, Kabila.

"Even though Kabila and his army of Tutsis came and killed us, we were obliged to forget that and join the Congo army. I fought in Katanga and Equateur [provinces]. That's how we liberated Congo," he said.

Kabila's recruitment of Rwandan Hutu forces and the genocidaires further infuriated Kagame in Rwanda, and opened the way to a prolonged war that drew in the Angolans and Zimbabweans on Congo's side while Uganda and Burundi lined up alongside Rwanda.

Plundering Congo's diamonds

The conflict also evolved into a matter of conquest and plunder as foreign armies ran lucrative sidelines in eastern Congo's diamonds, gold and coltan, a valuable component of mobile phones found in few other places in the world.

But for the Congolese the wars meant only more misery. That five years of slaughter after the second Rwandan invasion in 1998 cost so many lives that it has been called Africa's First World War. Millions died, mostly from disease and hunger, which means a high proportion of women and children. It was also a conflict marked by mass rape.

A shaky peace agreement in 2002 saw the withdrawal of foreign armies from Congo, although local rebel groups tied to the Rwandan government continued to control much of the east of the country.

It also brought Ngarambe and his men back to Rwanda's border where they began years earlier. But it did little to reassure Rwanda over its security as the Hutu extremists in eastern Congo launched a new armed group, the Democratic Forces for the Liberation of Rwanda (FDLR) led by men who had overseen the genocide in Rwanda. Ngarambe was among them, and they vowed to drive the Tutsis from power and "liberate" Rwanda.

In 2006, the UN security council declared the Hutu rebels "a serious threat to stability". Their menacing control of swaths of eastern Congo is also arguably the single most important factor in the continuing conflict in the region despite numerous peace deals.

The Hutu extremists on Rwanda's border unnerve the Tutsi-led government in Kigali which fears that they may one day gain enough strength to do what they threaten and finish off the genocide.

From Congo, Rwanda is seen as a belligerent power plundering its wealth. But Rwanda sees itself as besieged and fighting for the continued existence of the Tutsi people.

Which is why General Laurent Nkunda exists.

The renegade general

Nkunda is a former intelligence officer in the Rwandan army who went on to lead a rebel faction, now about 8,000 strong, in eastern Congo. His mission, he says, is to protect the local Tutsi population from the FDLR and other groups such as the Mai Mai.

But the Congolese government and the United Nations view Nkunda as a tool used by Rwanda to continue to destabilise eastern Congo.

The general is the walking stereotype of how Hutus portray Tutsis; tall and thin with a pinched face and pointed nose - all the attributes that led the Belgians to conclude that Tutsis were genetically superior because they more resembled white people physically than the shorter more rounded Hutus. Nkunda's hands are hardly spotless. In his campaign against the FDLR he has driven the Hutu population out of parts of the Congolese border province of North Kivu around Rutshuru and Masisi.

After a surge in rebel fighting in late 2007 against Congo's government troops, several mass graves were uncovered that were almost certainly the responsibility of Nkunda's fighters.

Nkunda has also been accused of war crimes and is the subject of an investigation by the international criminal court over the massacre of 160 people in Kisangani, prompting Mary Robinson, then UN Human Rights Commissioner, to call for his arrest in 2002 following the abduction and beating of two UN investigators by his troops.

In 2004, soldiers under Nkunda's command attacked the town of Bukavu. A Human Rights Watch report said they "went house to house raping and looting". Among the victims were teenagers and three girls of three years old.

Nkunda denies doing Rwanda's bidding. "I am Rwandaphone and I was in the Rwandan army and I have friends there. But that doesn't mean they tell me what to do or they give me any help at all," he said in a Guardian interview at his headquarters near Masisi. But there are tell-tale signs of the links. Some of his troops speak English, suggesting that they are Rwandan Tutsis who grew up in exile in Uganda. Nkunda's forces control villages and towns, and run courts, collect taxes and appoint priests in the local churches. Nkunda has won some influential friends abroad, including the support of Christian evangelists from the United States. He has taken to wearing a badge: "Rebels for Christ".

Rwanda's former foreign minister, Charles Murigande, concedes that the renegade general serves his country's interests as his forces are the only ones combating the FDLR. "Yes but we should also tell people that we are ready to fight the FDLR. We do not need anybody to do it for us. If somebody is doing it for us, well and good. But if that somebody is not there, the FDLR should know and everybody should know that we are ready to fight FDLR if that's what they want," he told the Guardian.

In 2006, the FDLR planned "Operation Amizero", an infiltration of Rwanda to forcibly recruit school-age Hutu children as fighters and to lay the ground for a military assault by hiding weapons caches and scouting out targets for sabotage. The operation collapsed after FDLR defectors warned the Rwandan government.

The failure of the rebels to launch a successful cross-border raid over several years has been taken as evidence that they are weakening. Certainly the FDLR is facing increasing numbers of defectors despite its policy of killing any it catches.

But the Tutsi leadership in Rwanda remembers its own history, and is worried about a longer game. Many of them were driven into exile as children, escaping with their families the massacres of Tutsis as the Belgians oversaw a "Hutu revolution" in the run-up to independence in 1962.

Three decades later, they were back in Rwanda as rebels fighting to overthrow the Hutu government. It took them just four years to seize power, a lesson not lost the Tutsi leadership.

Charles Murigande, who was two years old when his family fled Rwanda, was his country's foreign minister for five years of the conflict in Congo and remains in Paul Kagame's cabinet.

"There are people who say that Rwanda has a very strong army, while FDLR are numbered somewhere between 6,000 and 10,000. They say that is not a very grave threat to one of the best armies on the African continent. But I really get shocked when brilliant minds espouse that flawed reasoning," he said.

"I have never heard people using the same reasoning to discount the threat of al-Qaeda. I have never heard them saying that because America is such a powerful country, having the most powerful and sophisticated army in the world, therefore al-Qaeda is not a threat. It is as if killing Rwandans is OK, and that is very painful."